1. Field of the Invention
The present invention relates to a method and apparatus for performing a handover in a wireless communication system.
2. Description of the Related Art
In a wireless communication system, basic control channels are used for communication between a Base Station (BS) and a Mobile Station (MS). The basic control channels include a downlink preamble channel for delivering information needed for system synchronization, a downlink broadcast channel for delivering system information, and an uplink Ranging CHannel (RCH) for providing information used for an MS to enter a network.
Ranging is a process of periodically matching an accurate timing offset and adjusting power between a BS and an MS. The MS performs ranging during network entry, for example. The RCH carries information required for ranging. Although RCHs are typically allocated periodically, additional RCHs may be allocated dynamically when a large number of MSs are expected to enter the network.
FIGS. 1A and 1B illustrate exemplary frame structures to which RCHs are allocated, in a case of handover in a wireless communication system according to the related art.
FIG. 1A illustrates a frame structure for a BS A, and FIG. 1B illustrate a frame structure for BS B. Reference numerals 101 and 121 denote downlink frames for BS A and BS B, respectively. Reference numerals 103 and 123 denote uplink frames for BS A and BS B, respectively.
Referring to FIGS. 1A and 1B, BS A allocates an RCH 113 to the uplink frame 103 using a broadcast channel, SuperFrame Header (SFH) 105. When BS A determines to dynamically allocate additional RCHs, BS A may allocate RCHs 117 and 119 to the uplink frame 103 using MAPs 109 and 111 including dynamic resource allocation information.
The MAPs 109 and 111 that dynamically allocate the RCHs 117 and 119 are accessible to all users and the RCHs 117 and 119 are also available to all users.
In a situation such as handover, BS A may allocate a specific RCH to a specific MS. If an MS is to perform handover from BS B to BS A (i.e., a target BS), BS B may allocate an RCH 115 for the MS to use within BS A by unicasting a specific message, for example, a HandOver CoMmanD (HO-CMD) message 107 to the MS. The RCH 115 is determined through negotiation with BS A or under the control of a higher-layer controller.
Because the HO-CMD message 107 is unicast, all other MSs within BS A cannot receive information about the RCH 115 allocated to the handover MS.
In the illustrated case of FIG. 1, there are four RCHs 113, 115, 117, and 119. In general, the RCHs 113, 115, 117, and 119 are sequentially indexed from 0 to 3 in a frequency order or in a time order. The RCH indexing is performed individually by each of the BS and MSs according to their allocation information, not signaled between them.
Without knowledge of allocation information about the RCH 115 allocated by unicasting the HO-CMD message 107 to the handover MS, other MSs within BS A index the RCHs 113, 117, and 119 sequentially with 0, 1, and 2. On the other hand, BS A indexes the RCHs 113, 115, 117, and 119 sequentially with 0 to 3. As BS A and the MSs within BS A label the RCHs with different indexes, there may be an RCH identification mismatch between BS A and the MSs.